There are various systems and methods for making a different image reach each eye of a viewer. First are the systems which force the viewer to use glasses with different polarized, colored or shutter-based filters, or to use deflecting prisms or mirrors, and there are those which force the viewer to keep his/her head still in a certain position. Despite the obvious discomforts, these systems are quite widespread.
Systems and methods that do not require placing any device before the eyes of the viewers have been developed in an attempt to prevent said discomforts. Among such systems and methods are those herein referred to as three-dimensional systems, which use more than two reproduced images. Some of the systems or methods using two or very few images in reproduction are briefly discussed below.
Documents DE4123895 and EP0114406 describe methods and systems for three-dimensional reproduction for a large number of viewers using a small number of images. The different images are sequentially projected on a conventional diffusion screen and each viewer views through a shutter panel, preferably made with liquid crystal, located opposite and close to each viewer. Although discomforts occurring due to using glasses are prevented, the difficulty and high cost resulting from having to arrange a shutter panel before each viewer must be mentioned.
Documents GB2272597 and EP0656555A1 describe an image reproduction device using transparency, which is backlit by means of a device made up of a discrete assembly of light sources that are activated according to a signal sent from a detector detecting the location of the eyes of the viewers. Given the scarce depth of field of the detection and backlighting system, the eyes of the viewers must be located in one and the same plane, which is an important restriction.
Document EP2317367 describes an image reproduction device for reproducing images on a diffusion screen. The proposed system for detecting the eyes of the viewers is not capable of detecting the back and forth movements of viewers, and therefore the eyes of the viewers must be located in one and the same plane, which is an important restriction. Furthermore, tracking the head of the viewers requires mechanical movement of the optical focusing systems.
Document EP0576106 describes a device similar to that of GB2272597. The illuminating system differs from the one described in GB2272597 in that it includes a conventional image reproduction screen, which allows focusing on a large number of viewers with continuous tracking of their eyes. Nevertheless, due to its lack of depth of field, the eyes of the viewers must be located in one and the same plane, which in some cases may be a horizontal plane.
Document U.S. Pat. No. 5,976,017 describes a stereoscopic video game without glasses using a complex backlighting system because it needs several large-sized converging optical elements with focal lengths proportional to that size, whereby obtaining a system that takes up a large space and has a very small viewing angle. It furthermore uses a method for detecting the spatial location of the eyes of the viewers which needs as many cameras and as many light sources as there are viewers.
Document U.S. Pat. No. 5,016,282 and document EP0350957A both describe one and the same system capable of detecting eye movement and the gaze direction of a single viewer. The system is based, inter alia, on the fact that the polarized light used for illuminating the viewer is depolarized when it is reflected by any part of the viewer other than the cornea. In contrast, the rays reflected by the cornea conserve their polarization, given that the cornea acts like a polished surface. The system simultaneously captures two identical images of both the cornea and the retina from one and the same point, for which purpose it requires a special camera within which there are polarizing filters, discrimination prisms and image detecting elements. The system uses two different illuminating systems in the infrared spectrum, one close to the optical axis of the camera and the other one away from said optical axis, with two different wavelengths and frequency cut-off filters. The system described in this document is designed for capturing the gaze direction of a single viewer and is not valid for stereoscopic reproduction without glasses with multiple viewers.
Document US2002/0051118A describes a system for projecting an image on the retina of a single viewer, for which purpose it uses a detector for detecting the gaze direction of the viewer, which is not described in detail, and for stereoscopic reproduction it uses glasses.
Document U.S. Pat. No. 5,621,457 uses the light reflected in the cornea and in the retina by three different illuminating systems to find the gaze direction of a single viewer and is not valid for stereoscopic reproduction without glasses with multiple viewers.